Optical mid-span spectrum inversion, MSSI for short, is a powerful way to mitigate signal distortions due to non-linear effects in an optical fibre such as self-phase modulation, SPM for short, and cross-phase modulation, XPM for short. SPM occurs when the amplitude of a signal modulates the refractive index of the fibre glass and imposes as phase modulation on the signal itself, i.e. SPM introduces a chirp on the signal. XPM occurs when the amplitude modulation of a signal imposes a phase modulation on another signal, typically on another wavelength or on another optical polarization. SPM and XPM usually cause severe distortion on transmitted data, in particular if the data is encoded as phase states of the optical light. Both, i.e. SPM and XPM, are power dependent and can be neglected for low optical power level but as data rates as well as transmission distances increase, a high optical signal-to-noise ratio, OSNR for short, is needed and thus there is a need to increase the power. In MSSI applications the optical spectrum needs to be inverted in the middle of an optical link, where the link typically comprises at least one transparent optically amplified span before and after the optical spectrum inverter.
FIG. 1 shows a typical fibre optic link with optical spectrum inversion of one wavelength division multiplex channel, WDM channel for short, in the middle of the optical link. Typically the total link length is several hundreds or even thousands of kilometers long where each span comprises typically a length between 50 km to 100 km. By inverting the optical spectrum in the middle of the optical link, linear and non-linear phase distortion effects in the first half of the fibre could in principle be counteracted in the second half. Since non-linear transmission effects are the main limitation for better utilization of an optical fibre medium it is hard to find a possibility for overcoming capacity restrictions amongst other things.
So far only a few experimental works have addressed MSSI and all experiments used all-optical spectrum inverters that utilize four-wave mixing, FWM for short, in a non-linear optical medium. However, this concept requires a high optical power and cannot convert between the same wavelength at input and output, where the latter is very important in a WDM network. Further, using all-optical technologies for inversion is inherently very broadband and can thus invert several WDM channels at the same time. The absolute wavelengths are changed due to the inversion of the whole spectrum covered which makes this feature not desirable in practice.